Superlattice structure

ABSTRACT

A superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers is provided. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation-in-part of U.S. patent application Ser. No. 13/803,718, which was filed on 14 Mar. 2013, and which claims the benefit of U.S. Provisional Application No. 61/610,636, which was filed on 14 Mar. 2012 and U.S. Provisional Application No. 61/768,799, which was filed on 25 Feb. 2013, all of which are hereby incorporated by reference. Additionally, U.S. patent application Ser. No. 13/803,718 is a continuation-in-part of U.S. patent application Ser. No. 13/162,895, which was filed on 17 Jun. 2011, which is a continuation-in-part of U.S. patent application Ser. No. 12/987,102, which was filed on 8 Jan. 2011, and which claims the benefit of U.S. Provisional Application No. 61/293,614, which was filed on 8 Jan. 2010, all of which are hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of the Grant No. IIP-0839492 awarded by the National Science Foundation.

TECHNICAL FIELD

The disclosure relates generally to semiconductor devices, and more particularly, to a superlattice structure configured to reduce polarization effects of the semiconductor materials forming the devices.

BACKGROUND ART

In nitride based semiconductor materials and devices, including visible and ultraviolet (UV) light emitting diodes (LEDs), polarization effects play a dominant role causing strong built-in fields and spatial separation of electrons and holes. These polarization effects can negatively impact the performance of nitride-based visible and ultraviolet light emitting diodes. For example, FIGS. 1A-1C show illustrative band diagrams of a positive-intrinsic-negative (p-i-n) quantum well structure according to the prior art. In particular, FIG. 1A shows a band diagram of the structure without external bias and illumination; FIG. 1B shows a band diagram of the structure with the p-i-n field compensated by external bias; and FIG. 1C shows a band diagram of the structure with the total electric field compensated by external bias and intense optical excitation.

Polarization effects were evaluated for illustrative aluminum indium gallium nitride-based (Al_(x)In_(y)Ga_(1-x-y)N-based) multiple quantum well (MQW) structures. The MQW structures comprise an Al molar fraction in the quantum wells and barrier layers close to 20% and 40%, respectively, and In content in both the quantum wells and barriers of approximately 2% and 1%, respectively. The MQW structures comprise a total of three wells, each of which is two to four nanometers thick, separated by four five nanometer thick barriers.

Calculations indicated that the barriers and wells undergo tensions of 0.815% and 0.314%, respectively. These tensions correspond to piezoelectric charges at interfaces induced by this mismatch of −0.0484 coulombs per meter squared (C/m²) for the well and −0.0134 C/m² for the barrier. The polarization charge was calculated as −0.041 C/m² and −0.049 C/m² for the wells and barriers, respectively. The total electric field in the well for an alternating sequence of barriers and wells was found to be 1.2 Megavolts per centimeter (MV/cm). About fifty percent of the field was due to piezoelectric effect and the remaining fifty percent was caused by spontaneous polarization, both having the same direction. This corresponds to a 0.12 eV band bending in a one nanometer wide quantum well. Such band bending precludes using wide quantum wells in deep UV LEDs, which decreases the overall LED efficiency by limiting the MQW design optimization to very narrow (i.e., one to two nanometer thick) quantum wells.

SUMMARY OF THE INVENTION

Aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation.

A first aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second composition distinct from the first composition and a second polarization opposite the first polarization.

A second aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first composition having a first polarization for a first sub-layer of each of the plurality of periods; and selecting a second composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second composition is distinct from the first composition and the second polarization is opposite the first polarization.

A third aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization.

A fourth aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride composition and a first polarization, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second polarization opposite the first polarization.

A fifth aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first group III nitride composition having a first polarization for a first sub-layer of each of the plurality of periods, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and selecting a second group III nitride composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second group III nitride composition is distinct from the first group III nitride composition and the second polarization is opposite the first polarization.

A sixth aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization, wherein the first group III nitride-based composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization.

The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various aspects of the invention.

FIGS. 1A-1C show illustrative band diagrams of a p-i-n quantum well structure according to the prior art.

FIGS. 2A and 2B show illustrative structures according to the prior art and an embodiment, respectively.

FIG. 3 shows a conduction band diagram comparing a conduction band profile for a conventional quantum well and a conduction band profile for a quantum well according to an embodiment.

FIG. 4 shows another illustrative structure according to an embodiment.

FIG. 5 shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment.

FIG. 6 shows an illustrative light emitting device structure according to an embodiment.

FIG. 7 shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al_(x)Ga_(1-x)N alloy according to an embodiment.

FIG. 8 shows an illustrative chart for selecting an aluminum content of an AlGaN alloy to maintain a target transparency for a corresponding emitted wavelength according to an embodiment.

FIG. 9 shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment.

FIG. 10 shows possible molar fractions of indium and aluminum in an AlInGaN layer to achieve zero total polarization for the AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment.

FIG. 11 shows contour lines of polarization for various combinations of molar fractions of indium and aluminum in an AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment.

FIG. 12 shows an illustrative flow diagram for fabricating a circuit according to an embodiment.

It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation. As used herein, unless otherwise noted, the term “set” means one or more (i.e., at least one) and the phrase “any solution” means any now known or later developed solution.

Turning to the drawings, FIGS. 2A and 2B show illustrative structures 2, 10 according to the prior art and an embodiment, respectively. As illustrated in FIG. 2A, structure 2 includes a superlattice layer 4, which includes a plurality of repeating sub-layers 6A-6C. Each sub-layer 6A-6C can be separated from another sub-layer by a second set of sub-layers 8A-8B in the superlattice layer 4. Superlattice structure 4 can be configured to perform any type of function as part of a device incorporating structure 2. For example, sub-layers 6A-6C can comprise a set of quantum wells and sub-layers 8A-8B can comprise a set of barriers. In this case, superlattice layer 4 can comprise a multiple quantum well structure.

As shown in FIG. 2B, an embodiment of the invention provides a structure 10 including a superlattice layer 12 that is configured, for example, to reduce polarization effects. In particular, the superlattice layer 12 includes multiple periods 14A-14C, each of which includes two or more sub-layers 16, 18 having different compositions. Adjacent sub-layers 16, 18 in each period 14A-14C are configured to have polarizations (e.g., built-in electric fields) that at least partially cancel one another. For example, sub-layer 16 can comprise a spontaneous polarization having an opposite sign as a spontaneous polarization of sub-layer 18. Similarly, sub-layer 16 can comprise a strain-induced polarization having an opposite sign of a strain-induced polarization of sub-layer 18. Still further, one type of polarization in sub-layer 16 can have an opposite sign of another type of polarization in sub-layer 18, thereby reducing the net polarization present due to a combination of multiple types of polarizations (e.g., spontaneous and strain-induced).

In an embodiment, sub-layer 16 can comprise a positive or negative spontaneous polarization, while sub-layer 18 comprises the other of the positive or negative spontaneous polarization. In a more particular embodiment, the absolute values of the spontaneous polarizations of sub-layers 16, 18 are substantially equal, so that the net spontaneous polarization for the period 14A-14C is close to zero. In another embodiment, sub-layer 16 can comprise a strain-induced (e.g., piezoelectric) polarization due to stretching or compression, while sub-layer 18 comprises a strain-induced polarization due to the other of stretching or compression. In a more particular embodiment, the absolute values of the strain-induced polarizations of sub-layers 16, 18 are substantially equal, so that the net strain-induced polarization for the period 14A-14C is close to zero. It is understood that the respective spontaneous and/or strain-induced polarizations of sub-layers 16, 18 can be configured to only partially reduce the net spontaneous and/or strain-induced polarization for the period 14A-14C.

In still another embodiment, the spontaneous and/or strain-induced polarization of one sub-layer 16, 18 is configured to at least partially compensate the other of the spontaneous and/or strain-induced polarization of the other sub-layer 16, 18. For example, sub-layer 16 can comprise a spontaneous polarization of a first sign, and sub-layer 18 can comprise a strain-induced polarization of the opposite sign. In this case, the net polarization for the period 14A-14C will be reduced due to the two types of polarizations of the sub-layers 16, 18 compensating one another.

The various periods 14A-14C in superlattice layer 12 can be separated from one another by a set of additional sub-layers 20A-20B. In an embodiment sub-layers 20A-20B comprise inactive layers having no polarization. In another embodiment, each period 14A-14C comprises a quantum well, while each sub-layer 20A-20B comprises a barrier. In this case, superlattice layer 12 comprises a multiple quantum well structure. The periods (e.g., quantum wells) 14A-14C in superlattice layer 12 can be wider than the conventional sub-layers (e.g., quantum wells) 6A-6C. For example, in an embodiment, the width of superlattice layer 12 can be greater than two nanometers. In a more particular embodiment, the width of superlattice layer 12 is between approximately three nanometers and eight nanometers. In particular, periods 14A-14C will comprise a much smaller polarization field than that of a conventional sub-layer 6A-6C of a similar width. As a result, the detrimental confined Stark effect is avoided, which separates electrons and holes within a quantum well and prevents efficient radiative recombination.

FIG. 3 shows a conduction band diagram comparing a conduction band profile 22 for a conventional quantum well 6A (FIG. 2A) and a conduction band profile 24 for a quantum well 14A (FIG. 2B) according to an embodiment. As illustrated, the conduction band profile 24 comprises a more shallow profile than that of the conduction band profile 22. As a result, electrons in quantum well 14A can spread out within the quantum well 14A more than the electrons in quantum well 6A, providing for a more efficient radiative recombination.

Returning to FIG. 2B, superlattice 12 can perform any function as part of a device formed using structure 10. To this extent, superlattice 12 is located between a first layer 26 and a second layer 28 of the structure 10. In an illustrative embodiment, first layer 26 and second layer 28 can be formed from two dissimilar materials (e.g., two dissimilar nitride based semiconductor materials), and superlattice 12 can be graded in such a manner that it compensates (e.g., reduces) strain exerted by the dissimilar materials of layers 26, 28. For example, the lattice structure of each sub-layer 16, 18 of superlattice 12 can gradually change from a lattice structure similar to first layer 26 to a lattice structure similar to second layer 28.

While periods 14A-14C are each shown including two sub-layers 16, 18. It is understood that each period 14A-14C can include any number of sub-layers 16, 18. Similarly, while superlattice layer 12 is shown including three periods 14A-14C, it is understood that superlattice layer 12 can include any number of two or more periods 14A-14C. For example, FIG. 4 shows another illustrative structure 30 according to an embodiment. Structure 30 includes a superlattice layer 32, which comprises four periods 34A-34D that are separated by three sub-layers 36A-36C. Each period 34A-34D is formed by a set of six sub-layers of alternating compositions and polarizations. To this extent, each sub-layer of each period 34A-34D is immediately adjacent to one or two sub-layers having a different composition and an opposite polarization (e.g., spontaneous and/or strain-induced as described herein). In this manner, the periods 34A-34D can be made even wider than the conventional sub-layers 6A-6C of the prior art with smaller polarization fields than a conventional sub-layer 6A-6C of a similar thickness.

In an embodiment, structures 10 (FIG. 2B) and 30 (FIG. 4) can comprise nitride-based heterostructures. In a more specific embodiment, the structures 10, 30 comprise group III nitride-based heterostructures. In this case, the periods 14A-14C, 34A-34D of each structure 10, 30, respectively, each can be formed of group III nitride materials. Group III nitride materials comprise one or more group III elements (e.g., boron (B), aluminum (Al), gallium (Ga), and indium (In)) and nitrogen (N), such that B_(W)Al_(X)Ga_(Y)In_(Z)N, where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitride materials include AlN, GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBN with any molar fraction of group III elements. In an even more specific embodiment, the sub-layers described herein are quaternary or ternary group III nitride sub-layers, such as such as AlInN, AlGaN, InGaN, or AlInGaN. For further strain and/or polarization reduction, one or more sub-layers 16, 18 forming each period 14A-14C, 34A-34D can be doped. The sub-layers 16, 18 can be doped p-type or n-type. Furthermore, a sub-layer 16, 18 can comprise a monolayer.

FIG. 5 shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment. As illustrated, the calculated electric field drops to zero and goes negative as the indium molar fraction exceeds 0.7. In an illustrative embodiment, each sub-layer 16, 18 comprises AlInN with differing molar fractions of In. For example, sub-layer 16 can comprise an In molar fraction of approximately 0.65, which results in a calculated electric field of approximately 0.5 MV/cm, and sub-layer 18 can comprise an In molar fraction of approximately 0.77, which results in a calculated electric field of approximately −0.5 MV/cm. In this manner, the electric fields of both sub-layers 16, 18 can substantially cancel one another.

The superlattices 12, 32 described herein can be implemented as part of structures 10, 30 utilized for various types of devices, e.g., which are fabricated using semiconductor materials where polarization effects play a role. A superlattice 12, 32 described herein can be utilized as, for example, a multiple quantum well, an integral part of an ohmic and/or Schottky contact, a cladding layer, a buffer layer, a barrier layer, and/or the like, for the device. In an illustrative embodiment, structure 10 comprises a p-type contact including superlattice 12 and a metal layer 26 located thereon.

A structure 10, 30 described herein can be implemented as part of, for example, a light emitting device, such as a light emitting diode (LED), a superluminescent diode, or a laser. The light emitting device can comprise a visible light emitting device, an ultraviolet light emitting device, and/or the like. In this case, the light emitting device can include one or more superlattices as cladding layer(s), ohmic contact(s), and/or the like. In a more particular embodiment, the superlattice is formed as part of an ohmic contact for an ultraviolet light emitting device where a top p-type contact layer (e.g., layer 26 of FIG. 2B) of the ohmic contact, which is transparent to ultraviolet radiation, is located directly on the superlattice 12, 32. In a still more particular embodiment, the top p-type contact layer comprises AlInN.

It is understood that any combination of one or more layers (or sub-layers) in a structure 10, 30 can be configured to be at least partially transparent (e.g., semi-transparent or transparent) to radiation, such as ultraviolet radiation. As used herein, a layer is at least partially transparent to ultraviolet radiation if it allows more than approximately 0.001 percent of the ultraviolet radiation to pass there through. In a more particular embodiment, an at least partially transparent layer is configured to allow more than approximately five percent of the ultraviolet radiation to pass there through. In an embodiment, the at least partially transparent layer(s) are configured to be at least partially transparent to ultraviolet radiation emitted by the structure 10, 30. For example, the at least partially transparent layer(s) can be configured to be at least partially transparent to ultraviolet radiation in a range including the peak emission wavelength of the structure 10, 30 and at least five nanometers above and/or below the peak emission wavelength.

The layer(s) at least partially transparent to the ultraviolet radiation can be formed using any solution. For example, a transparent layer can comprise a p-type layer formed of a group III nitride material described herein. Illustrative at least partially transparent group-III nitride materials include AlGaN, AlInGaN, boron-containing alloys (GaBN, AIBN, AlGaBN, AlInGaBN, InGaBN, and/or the like), and/or the like. Furthermore, the at least partial transparency of a layer can be achieved using any solution. For example, at least partial transparency can be achieved in materials with bandgaps smaller than a photon energy of the ultraviolet radiation due to tunneling, thermionic transport via impurity states, and/or the like.

Similarly, it is understood that any combination of one or more layers in a structure 10, 30 can be configured to reflect ultraviolet radiation. As used herein, a layer is reflective of ultraviolet radiation when it reflects more than approximately five percent of the ultraviolet radiation. In an embodiment, the reflective layer(s) are configured to reflect ultraviolet radiation emitted by the structure 10, 30. For example, the reflective layer(s) can be configured to reflect ultraviolet radiation in a range including the peak emission wavelength of the structure 10, 30 and at least five nanometers above and/or below the peak emission wavelength.

The ultraviolet reflective layer(s) can be formed using any solution. For example, a reflective layer can comprise a metal coating formed of Al, Rhodium (Rh), enhanced Al, enhanced Rh, Gold (Au), Aluminum Silicon Monoxide (AlSiO), Aluminum Magnesium Fluoride (AlMgF₂), and/or the like. Furthermore, the reflectivity of a layer can be achieved using any solution. For example, reflectivity can be achieved by the formation of a reflecting photonic crystal, a distributed Bragg reflector (DBR) structure, and/or the like.

The at least partially ultraviolet transparent and/or reflective layer(s) can comprise any of various layers of a structure 10, 30 based on a desired operating configuration for the structure 10, 30. For example, a structure 10, 30 can include an at least partially ultraviolet transparent contact. Such a contact can comprise, for example, a p-type at least partially ultraviolet transparent layer 26 (FIG. 2B) and an interlayer, such as an at least partially ultraviolet transparent superlattice 12 (FIG. 2B), for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like. Similarly, a structure 10, 30 can include an ultraviolet reflecting contact, which is configured to reflect a desired amount of the ultraviolet radiation generated by the structure 10, 30. Such a reflecting contact also can include, for example, a p-type ultraviolet reflective layer 26 and a superlattice 12 for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like.

A structure 10, 30 can include various other layers, which are at least partially ultraviolet transparent and/or ultraviolet reflective, such as a p-type superlattice 12, 32, an electron blocking layer located between a superlattice 12, 32 and a multiple quantum well structure, and/or the like. In each case, the at least partially ultraviolet transparent and/or ultraviolet reflective layer can be formed using any type of material. In an embodiment the at least partially ultraviolet transparent and/or ultraviolet reflective layer is formed using a group-III nitride material, such as boron-containing layers.

FIG. 6 shows an illustrative light emitting device structure 40 according to an embodiment. As illustrated, the device structure 40 comprises an n-type contact layer 50 adjacent to a radiation generating structure 52. Radiation generating structure 52 can comprise any type of structure, such as a multiple quantum well structure, for generating any type of radiation, such as ultraviolet light. Furthermore, device structure 40 includes a p-type contact layer 54 on an opposing side of the radiation generating structure 52 as the n-type contact layer 50.

The device structure 40 further includes a superlattice layer 12, which can be formed as described herein. Superlattice layer 12 is shown located on the same side of the radiation generating structure 52 as the p-type contact layer 54. In an embodiment, the superlattice layer 12 is at least partially transparent to radiation generated by radiation generating structure 52. It is understood that superlattice layer 12 is only illustrative of the types of superlattices that can be included in the device structure 40. For example, the device structure 40 could include superlattice 32 and/or a variant of the superlattices shown herein.

The device structure 40 also can include an electron blocking layer 56, which can be located between the superlattice layer 12 and the radiation generating structure 52. In an embodiment, the electron blocking layer 56 has a thickness in a range between approximately two and approximately one hundred nanometers. The electron blocking layer 56 can comprise a p-type composition having a larger band gap than the barrier(s) located within the superlattice layer 12, which can result in an improved transparency of the electron blocking layer to radiation generated by the radiation generating structure 52. Furthermore, the electron blocking layer 56 can comprise a graded composition, which can be configured to decrease a resistance of the electron blocking layer 56. For example, the electron blocking layer 56 can have a graded doping that increases or decreases by approximately 10⁴ cm⁻³, e.g., between approximately 10¹⁶ and approximately 10²⁰ cm⁻³. Alternatively, the electron blocking layer 56 can have a homogeneous doping within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³.

The device structure 40 can include a contact 60. Contact 60 can comprise any type of contact. In an embodiment, the contact 60 comprises a p-type metal contact, such as a Schottky contact, a leaky Schottky contact, a rectifying contact, and/or the like. In a more specific embodiment, the contact 60 at least partially reflects the radiation generated by the radiation generating structure 52 and can be formed from, among other things, aluminum, enhanced aluminum, aluminum silicon monoxide, aluminum magnesium fluoride, rhodium, enhanced rhodium, gold, and/or the like. In another more specific embodiment, the contact 60 is at least partially transparent to the radiation generated by the radiation generating structure 52 and can be formed from, among other things, a metallic superlattice, in which each layer is at least partially transparent to the radiation. In either case, the contact 60 can be directly adjacent to a transparent adhesion layer 58. The transparent adhesion layer 58 can be configured to improve ohmic properties of the contact 60 and promote adhesion of the contact 60 to a surface of the semiconductor (e.g., layer 54). In an embodiment, the transparent adhesion layer 58 is formed of nickel. However, it is understood that transparent adhesion layer 58 can be formed of any suitable material, including Nickel oxyhydroxide (NiOx), Palladium (Pd), Molybdenum (Mo), Cobalt (Co), and/or the like.

The various layers in the device structure 40 can be formed using any type of materials. In an embodiment, the device structure 40 comprises a group III nitride-based heterostructure, in which one or more of the layers 50, 56, 12, and 54 and radiation generating structure 52 are formed of various group III nitride materials using any solution. Additionally, contact 60 can be implemented without a transparent adhesion layer 58, and be formed of one or more layers of metal, such as for example, one or more layers of titanium, aluminum, gold, chromium, nickel, platinum, lead, rhodium, and/or the like.

In an embodiment, one or more of the contacts 50, 54, 60 comprises graphene, which can be configured to be transparent to radiation generated by the radiation generating structure 52 and very conductive. For example, the p-type contact layer 54 to the superlattice layer 12 and/or contact 60 can be at least partially formed of p-type graphene. Similarly, the n-type contact layer 50 can be at least partially formed of n-type graphene. In an embodiment, a contact 50, 54, 60 comprises a graphene composite contact, which includes a graphene sub-layer adjacent to a thin sub-layer of metal, which can improve current spreading in the contact 50, 54, 60. In a further embodiment, the graphene composite contact is at least partially transparent to the radiation generated by the radiation generating structure 52. It is understood that the device structure 40 can include one or more layers, such as transparent adhesion layer 58 and/or contact 60, adjacent to a contact formed of graphene, such as contact 54, which are configured to improve light extraction from the device structure 40, e.g., via a textured surface.

In an embodiment, a structure described herein can include one or more layers having a composition selected such that the layer has a transparency of at least a target transparency to radiation, such as ultraviolet radiation, of a target set of wavelengths. The layer can comprise, for example, a p-type contact layer 54 (FIG. 6), an electron blocking layer 56 (FIG. 6), a superlattice layer 12 (FIG. 6), and/or the like. For example, a layer can be a group III nitride-based layer, which is composed of Al_(x)Ga_(1-x)N where the aluminum molar fraction (x) is sufficiently high in some domains of the layer to result in the layer being at least partially transparent to ultraviolet radiation. In an embodiment, the layer can comprise a superlattice layer located in an emitting device configured to emit radiation having a dominant wavelength in the ultraviolet spectrum, and the composition of at least one sub-layer in each period of the superlattice layer is configured to be at least partially transparent to ultraviolet radiation having a target wavelength corresponding to the ultraviolet radiation emitted by the emitting device.

In an embodiment, the sub-layer has a thickness in a range between approximately one and approximately one thousand nanometers. Furthermore, the sub-layer can have a graded doping that increases or decreases by approximately 10⁴ cm⁻³, e.g., between approximately 10¹⁶ and approximately 10²⁰ cm⁻³. Alternatively, the sub-layer can have a homogeneous doping within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³. The doping can be any type of doping. For example, the doping can be: modulation doping; unintentional doping by impurities from one or more of oxygen, hydrogen, and magnesium; a dopant, such as magnesium and/or carbon, diffused from another doped layer or present in the growth chamber as residual elements; and/or the like. In an embodiment, one or more sub-layers can be co-doped with magnesium and carbon, where both the carbon and magnesium doping levels are within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³, but the combined concentration of the dopants does not exceed approximately 10²⁰ cm⁻³. In another embodiment, the doping can alternate between two or more dopants. For example, a sub-layer can include carbon doping, while the adjacent sub-layer(s) can include magnesium doping.

An amount of transparency of a short period superlattice (SPSL) can be approximated by computing the averaged band gap of the SPSL, and deducing average absorption coefficient of the SPSL. The absorption coefficients depend on an absorption edge of the semiconductor material, which for materials formed of an AlGaN alloy, is a function of the molar fractions of the Al_(x)Ga_(1-x)N semiconductor alloy.

In an embodiment, the target transparency for the material is at least ten times more transparent than the least transparent layer of material in the structure (e.g., GaN for a group III nitride-based device). In this case, an absorption coefficient of the semiconductor layer can be on the order of 10⁴ inverse centimeters or lower. In this case, a one micron thick semiconductor layer will allow approximately thirty-six percent of the ultraviolet radiation to pass there through.

FIG. 7 shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al_(x)Ga_(1-x)N alloy according to an embodiment. In order to maintain an absorption coefficient of the semiconductor layer at orders of 10⁴ inverse centimeters or lower, the content of aluminum in an SPSL barrier layer can be chosen based on the corresponding target wavelength or range of wavelengths. For example, for a target wavelength of approximately 250 nanometers, the aluminum molar fraction can be approximately 0.7 or higher, whereas for a target wavelength of approximately 300 nanometers, the aluminum molar fraction can be as low as approximately 0.4. FIG. 8 shows an illustrative chart for selecting an aluminum content of an Al_(x)Ga_(1-x)N alloy to maintain a target transparency for a corresponding emitted wavelength, λ, according to an embodiment. In this case, the target transparency corresponds to an absorption coefficient of the semiconductor layer on the order of 10⁴ inverse centimeters. Note that in FIG. 8, the dependence of x=x(λ) is linear, with x=C·λ+B, where C=−0.0048 nm⁻¹, and B=1.83.

In an embodiment, one or more sub-layers of the SPSL can have a graded composition. For example, a sub-layer of the SPSL can be formed of an Al_(x)Ga_(1-x)N alloy, where the aluminum molar fraction, x, is continually varied in the vertical direction of the sub-layer.

In an embodiment, a device can include one or more layers with lateral regions configured to facilitate the transmission of radiation through the layer and lateral regions configured to facilitate current flow through the layer. For example, the layer can be a short period superlattice, which includes barriers alternating with wells. In this case, the barriers can include both transparent regions, which are configured to reduce an amount of radiation that is absorbed in the layer, and higher conductive regions, which are configured to keep the voltage drop across the layer within a desired range. As used herein, the term lateral means the plane of the layer that is substantially parallel with the surface of the layer adjacent to another layer of the device. As described herein, the lateral cross section of the layer can include a set of transparent regions, which correspond to those regions having a relatively high aluminum content, and a set of higher conductive regions, which correspond to those regions having a relatively low aluminum content.

The set of transparent regions can be configured to allow a significant amount of the radiation to pass through the layer, while the set of higher conductive regions can be configured to keep the voltage drop across the layer within a desired range (e.g., less than ten percent of a total voltage drop across the structure). In an embodiment, the set of transparent regions occupy at least ten percent of the lateral area of the layer, while the set of higher conductive regions occupy at least approximately two percent (five percent in a more specific embodiment) of the lateral area of the layer. Furthermore, in an embodiment, a band gap of the higher conductive regions is at least five percent smaller than the band gap of the transparent regions. In a more particular embodiment, the transparent regions comprise a transmission coefficient for radiation of a target wavelength higher than approximately sixty percent (eighty percent in a still more particular embodiment), while the higher conductive regions have a resistance per unit area to vertical current flow that is smaller than approximately 10⁻² ohm·cm². As used herein, the term transmission coefficient means the ratio of an amount of radiation exiting the region to an amount of radiation entering the region.

The transparent and conductive regions can be formed using any solution. For example, a layer can be grown using migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). During the growth, inhomogeneities in the lateral direction of a molar fraction of one or more elements, such as aluminum, gallium, indium, boron, and/or the like, can be allowed in the layer. In an embodiment, such compositional inhomogeneities can vary by at least one percent.

In an embodiment, a light emitting device structure can include one or more structures configured to reduce an overall polarity of the structure. In embodiment, the structure can form a cladding layer, a p-type contact layer, and/or the like, of a light emitting device. In order to confine a polarization charge within a sub-layer, the sub-layer thicknesses can be larger than a Bohr radius of the carriers. Using a p-type contact layer as an illustrative example, the Bohr radius, R_(B), can be calculated for hole carriers. In this case, the Bohr radius is given by R_(B)=4πεℏ²/m_(h)e², where ε is the permittivity of the material, ℏ is the reduced Planck's constant, m_(h) is the hole rest mass, and e is the elementary charge. For the composition Al_(0.5)Ga_(0.5)N, the mass of an “average” hole is about four times the electron rest mass (m_(h)˜4m_(e)), the permittivity is approximately nine times the permittivity of free space (ε˜9ε₀), and the resulting Bohr radius, R_(B), is approximately 9/4 of the Bohr radius of hydrogen, R_(H), that is R_(B)˜1.2 nm. A group III semiconductor layer having a higher concentration of gallium will have a smaller hole mass (e.g., for GaN, m_(h)˜1.4). As a result, such a group III semiconductor layer can have a Bohr radius, R_(B)˜6×R_(H)=3.2 nm.

An AlGaN film deposited by MOCVD on a substrate formed of sapphire, SiC, Si, and/or the like, typically grows with its gallium face up. This growth corresponds to the growth direction of the film being [0001], the positive c-axis direction. However, growth of a heavily Mg-doped AlGaN layer by MOCVD can produce a negative c-axis direction (N-face growth) of AlGaN. The inversion of polarity can reduce an overall “average” polarity within a given sub-layer. To this extent, FIG. 9 shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment. As illustrated the layer includes a plurality of lateral domains, at least one of which is a nitride facing domain (N-face) and at least one of which is a gallium facing domain (Ga-face). As illustrated, the polarization (P_(S)) and electric field (E) vectors are inversed on either side of the boundary between the domains.

In an embodiment, polarization control of a group III nitride semiconductor layer, such as a sub-layer in a superlattice layer described herein, is accomplished using an alloy, such as Al_(x)In_(y)B_(z)Ga_(1-x-y-z)N, where 0≦x, y, z≦1 and 0≦x+y+z≦1. A group III nitride semiconductor layer deposited on another group III nitride semiconductor layer can exhibit both piezoelectric and spontaneous polarization. These polarizations can compensate one another, resulting in a lowering of a polarization of the deposited semiconductor layer.

For example, FIG. 10 shows possible molar fractions of indium and aluminum in an AlInGaN layer to achieve zero total polarization for the AlInGaN layer when grown over an AlN layer according to an embodiment. As illustrated, as an aluminum molar fraction increases in the AlInGaN alloy, a molar fraction for indium also needs to increase to achieve zero total polarization for the AlInGaN layer. In an embodiment, any combination of Al and In molar fractions, which result in a zero total polarization for the AlInGaN layer is selected. In a more particular embodiment, an Al_(0.4)In_(0.2)Ga_(0.4)N layer can be used to achieve zero total polarization for the AlInGaN layer.

In an embodiment, an alloy can be selected to provide a target polarization for the corresponding layer. The target polarization can be positive or negative, any magnitude within a range of polarization magnitudes, and/or the like. Continuing to use an AlInGaN layer grown on an AlN layer as an illustrative example, FIG. 11 shows contour lines of polarization for various combinations of molar fractions of indium and aluminum in an AlInGaN layer in an AlN/AlInGaN heterostructure according to an embodiment. As noted, the thick line corresponds to a substantially zero total polarization for the AlInGaN layer as shown in FIG. 10. For molar fractions of indium and aluminum having a higher relative indium content than that providing a zero total polarization (e.g., contour lines located above the zero total polarization line), a positive net polarization is present within the AlInGaN layer, which can result from piezoelectric polarization dominating over spontaneous polarization in the AlInGaN material. Conversely, for molar fractions of indium and aluminum having a higher relative aluminum content than that providing zero total polarization (e.g., contour lines located below the zero total polarization line), a negative net polarization is present within the AlInGaN layer, which can result from spontaneous polarization dominating over piezoelectric polarization in the AlInGaN material.

As illustrated by FIGS. 10 and 11, having a quaternary group III nitride semiconductor alloy, such as AlInGaN, allows for a thorough control of both the sign and magnitude of the net polarization in the semiconductor material. However, it is understood that FIGS. 10 and 11 are only illustrative of various embodiments, which can be used to control and affect polarization within a semiconductor layer of any type. For example, the addition of boron to the AlInGaN material also can further influence the resulting polarization. Similarly, the base layer can comprise another material, such as another group III nitride material, other than AlN. Furthermore, selection of and deposition of neighboring semiconductor layers can impact polarization within the semiconductor layer due to the presence of lattice mismatch stresses at the layer interfaces, which can result in increased piezoelectric polarization. The polarization within the semiconductor layer can be evaluated using expressions for piezoelectric and spontaneous polarization known in art of the group III nitride semiconductors.

Structures 10, 30 described herein can be incorporated as part of, for example, a transistor (e.g., a field effect transistor), a photodetector, a monolithic and/or optoelectronic integrated circuit, a metal-semiconductor diode, a p-n junction diode, a switch, and/or the like. In this case, the device can include one or more superlattices as buffer layer(s), barrier layer(s), contact layer(s), and/or the like. In a more particular embodiment, the periods of the superlattice layer are formed from AlInN.

While shown and described herein with respect to the fabrication of a superlattice layer, it is understood that an embodiment of the invention can be applied to the fabrication of a heterostructure comprising a set of quantum wells and a set of barriers. The various sub-layers shown and described herein can be formed using any solution. For example, the superlattice layers 12, 32 can be grown using a combination of metallo organic chemical vapor deposition (MOCVD) and/or migration enhanced MOCVD (MEMOCVD), in which each period in the superlattice layer 12, 32 requires at least two growth steps.

While shown and described herein as a method of designing and/or fabricating a structure and/or a corresponding semiconductor device including the structure, it is understood that aspects of the invention further provide various alternative embodiments. For example, in one embodiment, the invention provides a method of designing and/or fabricating a circuit that includes one or more of the semiconductor devices designed and fabricated as described herein (e.g., including one or more superlattice layers 12, 32).

To this extent, FIG. 12 shows an illustrative flow diagram for fabricating a circuit 126 according to an embodiment. Initially, a user can utilize a device design system 110 to generate a device design 112 using a method described herein. The device design 112 can comprise program code, which can be used by a device fabrication system 114 to generate a set of physical devices 116 according to the features defined by the device design 112. Similarly, the device design 112 can be provided to a circuit design system 120 (e.g., as an available component for use in circuits), which a user can utilize to generate a circuit design 122 (e.g., by connecting one or more inputs and outputs to various devices included in a circuit). The circuit design 122 can comprise program code that includes a device designed using a method described herein. In any event, the circuit design 122 and/or one or more physical devices 116 can be provided to a circuit fabrication system 124, which can generate a physical circuit 126 according to the circuit design 122. The physical circuit 126 can include one or more devices 116 designed using a method described herein.

In another embodiment, the invention provides a device design system 110 for designing and/or a device fabrication system 114 for fabricating a semiconductor device 116 by using a method described herein. In this case, the system 110, 114 can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the semiconductor device 116 as described herein. Similarly, an embodiment of the invention provides a circuit design system 120 for designing and/or a circuit fabrication system 124 for fabricating a circuit 126 that includes at least one device 116 designed and/or fabricated using a method described herein. In this case, the system 120, 124 can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the circuit 126 including at least one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer program fixed in at least one computer-readable medium, which when executed, enables a computer system to implement a method of designing and/or fabricating a semiconductor device as described herein. For example, the computer program can enable the device design system 110 to generate the device design 112 as described herein. To this extent, the computer-readable medium includes program code, which implements some or all of a process described herein when executed by the computer system. It is understood that the term “computer-readable medium” comprises one or more of any type of tangible medium of expression, now known or later developed, from which a copy of the program code can be perceived, reproduced, or otherwise communicated by a computing device. For example, the computer-readable medium can comprise: one or more portable storage articles of manufacture; one or more memory/storage components of a computing device; paper; and/or the like.

In another embodiment, the invention provides a method of providing a copy of program code, which implements some or all of a process described herein when executed by a computer system. In this case, a computer system can process a copy of the program code to generate and transmit, for reception at a second, distinct location, a set of data signals that has one or more of its characteristics set and/or changed in such a manner as to encode a copy of the program code in the set of data signals. Similarly, an embodiment of the invention provides a method of acquiring a copy of program code that implements some or all of a process described herein, which includes a computer system receiving the set of data signals described herein, and translating the set of data signals into a copy of the computer program fixed in at least one computer-readable medium. In either case, the set of data signals can be transmitted/received using any type of communications link.

In still another embodiment, the invention provides a method of generating a device design system 110 for designing and/or a device fabrication system 114 for fabricating a semiconductor device as described herein. In this case, a computer system can be obtained (e.g., created, maintained, made available, etc.) and one or more components for performing a process described herein can be obtained (e.g., created, purchased, used, modified, etc.) and deployed to the computer system. To this extent, the deployment can comprise one or more of: (1) installing program code on a computing device; (2) adding one or more computing and/or I/O devices to the computer system; (3) incorporating and/or modifying the computer system to enable it to perform a process described herein; and/or the like.

The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims. 

What is claimed is:
 1. A structure comprising: a first layer; and a superlattice layer having a first side immediately adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride composition and a first built-in electric field; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second built-in electric field opposite the first built-in electric field, wherein the first and second built-in electric fields substantially cancel one another.
 2. The structure of claim 1, wherein each of the plurality of periods includes at least one additional sub-layer, and wherein the sub-layers in each of the plurality of periods have built-in electric fields of alternating signs.
 3. The structure of claim 1, wherein the first group III nitride composition is an Al_(x)Ga_(1-x)N alloy.
 4. The structure of claim 3, wherein the second group III nitride composition is an AlInBGaN alloy, and wherein a composition of the AlInBGaN alloy is selected based on a target polarization for the second group III nitride composition.
 5. The structure of claim 1, wherein the superlattice layer further includes a plurality of third sub-layers separating each of the plurality of periods.
 6. The structure of claim 5, wherein each of the plurality of periods comprises a quantum well and each of the third sub-layers comprises a barrier.
 7. The structure of claim 1, wherein the first layer comprises an electron blocking layer, the structure further comprising a multiple quantum well structure adjacent to a second side of the superlattice layer, wherein the second side is opposite the first side.
 8. The structure of claim 1, wherein the first layer and the superlattice layer form at least one of: a cladding layer or a p-type contact.
 9. The structure of claim 1, wherein the superlattice layer is transparent to ultraviolet radiation, and wherein the ultraviolet radiation is in a range including a peak emission wavelength of ultraviolet radiation emitted by the structure.
 10. The structure of claim 1, further comprising a second layer immediately adjacent to a second side of the superlattice layer opposite the first side, wherein the compositions of each of the plurality of periods changes from a composition having a lattice structure similar to the first layer for a period located adjacent to the first side to a composition having a lattice structure similar to the second layer for a period located adjacent to the second side.
 11. The structure of claim 1, wherein the first layer is formed of a group III nitride material comprising indium.
 12. The structure of claim 11, wherein the first and second group III nitride compositions include differing molar fractions of indium.
 13. The structure of claim 1, wherein the structure comprises a heterostructure device configured to operate as at least one of: a light emitting diode, a superluminescent diode, or a laser.
 14. A method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side immediately adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first group III nitride composition having a first built-in electric field for a first sub-layer of each of the plurality of periods, wherein the first group III nitride composition is selected such that the first sub-layer is at least partially transparent to ultraviolet radiation of a target wavelength; and selecting a second group III nitride composition having a second built-in electric field for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second group III nitride composition is distinct from the first group III nitride composition and the second built-in electric field is opposite the first built-in electric field, wherein the first and second built-in electric fields substantially cancel one another.
 15. The method of claim 14, wherein the first group III nitride composition includes aluminum.
 16. The method of claim 15, wherein the first group III nitride composition is an Al_(x)Ga_(1-x)N alloy, and wherein the second group III nitride composition is an AlInBGaN alloy, and wherein a composition of the AlInBGaN alloy is selected based on a target polarization for the second group III nitride composition.
 17. The method of claim 14, further comprising fabricating at least one device according to the structure design.
 18. The method of claim 17, wherein the fabricating includes growing the first sub-layer such that a composition of the first sub-layer varies along lateral dimensions of the first sub-layer such that a lateral cross section of the first sub-layer includes: a set of transparent regions, each transparent region having a transmission coefficient for the target wavelength greater than or equal to approximately sixty percent, wherein the set of transparent regions are at least ten percent of an area of the lateral cross section of the first sub-layer; and a set of higher conductive regions occupying a sufficient area of the area of the lateral cross section of the first sub-layer and having an average resistance per unit area to a vertical current flow resulting in a total voltage drop across the superlattice layer of less than ten percent of a total voltage drop across the structure.
 19. The method of claim 18, wherein each of the set of higher conductive regions comprises a molar fraction of aluminum providing a band gap at least five percent smaller than a band gap of the set of transparent regions.
 20. A group III nitride-based device comprising: a p-type contact comprising: a first p-type metal layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first built-in electric field; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second built-in electric field opposite the first built-in electric field, wherein the first built-in electric field and the second built-in electric field substantially cancel one another and comprise at least one of: a strain-induced polarization or a spontaneous polarization for the corresponding composition, and wherein the p-type contact is transparent to ultraviolet radiation. 